Types of retinoblastoma

There are 2 different types of retinoblastoma. The type that develops depends on where and when the RB1 mutation occurs and whether the mutation is inherited (passed to a child from a parent).

Non-heritable retinoblastoma (also called sporadic retinoblastoma) happens by chance. About half the number of children with retinoblastoma have non-heritable retinoblastoma.

Children are born with 2 normal copies of the RB1 gene. A mutation of both copies of the RB1 gene in a retinoblast cell causes a retinoblastoma tumour to form in the eye.

Sometimes non-heritable retinoblastoma can happen even though the RB1 genes are normal. In this case, there are too many copies of the gene called MYCN. When there are too many copies of a gene, itʼs called an amplification. The amplification of the MYCN gene causes retinoblastoma that grows quickly and usually occurs in children around the age of 4 months old.

Children with non-heritable retinoblastoma develop one tumour in one eye (called unilateral retinoblastoma). They will not pass the RB1 mutation to their children.

Heritable retinoblastoma is the result of a mutation in the RB1 gene that is either inherited, the mutation happens in the egg or sperm before or soon after conception (called a germline mutation), or when the embryo is developing. About half of children with retinoblastoma have heritable retinoblastoma.

In all cases of heritable retinoblastoma, a child is born with one copy of the RB1 mutation in all or a portion of all cells of the body. This increases the risk of developing cancer in other parts of the body later in life. A mutation of the second copy of the RB1 gene then occurs in retinoblasts and causes retinoblastoma to form. In about 10% of children with heritable retinoblastoma, the mutation of the second copy of the RB1 gene was inherited from a parent. When this happens, it’s called familial retinoblastoma.

Children with heritable retinoblastoma can have more than one tumour, and these tumours can develop in one or both eyes (called bilateral retinoblastoma).

Children with heritable retinoblastoma can pass the RB1 mutation on to their children. They also have a greater risk of developing a tumour in the pineal gland in the brain (called a pineoblastoma). When this happens, itʼs referred to as trilateral retinoblastoma.